The application of nanotechnology to drug transport requires an understanding of how nanoscale materials interact with and cross cellular membranes. The principle mechanisms debated in the literature for internalization of nanoscale materials into cells include energy-dependent endocytosis, energy-independent membrane translocation, and energy-dependent formation of nanoscale membrane holes. This research program will test these three mechanistic hypotheses for an important class of nanoscale materials, dendrimers, amine-terminated polycationic polymers, and cell-penetrating peptides, that have relevance to drug delivery and cell transfection. In addition to assessing the relative significance of these three internalization mechanisms, the research program will also explore the key physical interactions between the nanoscale materials and the membrane that cause cell membrane permeability. This is important regardless of the mechanism used for internalization since induction of permeability could lead to poor selectivity in drug delivery applications and/or toxicity. Developing an understanding of the physical parameters that lead to cell membrane permeability, and exploring the possible formation of nanoscale membrane holes as a microscopic mechanism of permeabiliy, is important for the rational use of nanotechnology for the application of drug transport. The specific aims of this research are: 1) assessment of the membrane interaction and transport mechanism of a class of nanoscale materials including dendrimers and polycationic polymers 2) quantification of the relationship between hole formation in lipid bilayers and the size, surface chemistry, and charge of the nanoscale materials 3) determination of the extent and nature of nanoscale hole formation in living cell membranes induced by the nanoscale materials. This work is relevant to public health because it will provide the basic understanding needed to design delivery platforms for drug or gene therapy. Properly designed systems will lead to therapeutics with signficantly reduced side effects.